Internal combustion engines are frequently of the piston-type. The piston(s) of these engines are subjected to extreme forces, frictional wear, and high temperature. In addition, the shape and size of the piston greatly affects the performance of the engine. So that the piston and the engine perform optimally, the piston should satisfy several criteria.
First, the piston should be light-weight. For among other reasons, reducing the weight of the piston reduces inertial forces generated by the piston as its moves within the engine. Generally, for the piston to be light-weight, it must be thin walled to reduce its mass, and be constructed of a low-density material.
The height of the land portion of the piston (i.e. that portion of the piston above the piston ring) should be small. This reduced height increases the compression ratio, which results in increased engine performance. In addition, the shorter land results in a smaller crevice or "squish" volume, causing a reduction in the amount of unburned fuel, improving exhaust emissions. For the piston to have a short land, it is normally necessary for the piston material to maintain its hardness even at temperatures above about 350.degree. C. so as not to thermally fuse the ring(s) thereto and so that the corner of the land does not yield or deform at high temperature (i.e. above about 350.degree. C.).
Not only must the land portion of the piston not deform for the reasons described above, but the other portions of the piston, such as the skirt, must also not deform during the piston's use. This normally requires that the piston be thick-walled and be constructed of a material which retains a high Young's modulus even at high temperatures (i.e. above about 350.degree. C. on the piston's top surface).
In sum, the piston must have a high fatigue strength, a high proof strength, and a high hardness at high temperatures, and yet be constructed from a material which has a low density and allows the piston to be of a thin-walled construction. Further, the piston material must on the one hand provide high strength and hardness, and yet must be yieldable if the piston is to be forged (as opposed to cast or machined, both of which processes increase the cost of manufacture of the piston). To date, no material and piston configuration has satisfied all of these criteria.
As one attempt at satisfying these criteria, it is known to construct a piston having a head portion which is clad with a different material than a material which clads a skirt portion of the piston (such as by having the first cladding material comprise aluminum and the other comprise a compound layer made of aluminum mixed with fibers of material). The claddings comprising different materials are joined together by forging.
This arrangement is disadvantageous because insufficient joining strength is provided at the interface between the joined materials. Generally, this is now believed to be, in part, due to the fact that insufficient slip occurs between the two materials during forging. As a result, an oxide film on the surfaces is not destroyed, this film inhibiting strong bonding between the materials. As one means for increasing this bonding strength, fiber reinforcement may be used. This tends to create stress concentrations to occur on the interfaces between the matrix and the reinforcing fibers or material, such that an insufficient fatigue strength at high temperatures is the result. Also, this method of manufacture increases the manufacturing cost, and generally can not be used when its is desired that only a small portion of the piston (such as the area about the piston ring groove(s)) be formed of a different material.
In a second arrangement, it is known to make a two-layer composition by powder-forming quenched powder aluminum matrices (powder metal) of a common composition, each layer having a different ratio of included ceramic powder. The two-layer composition is then heat-pressed to form a body. The body is then heat-forged into form a piston, with the head portion containing a higher ratio of ceramic powder and the skirt portion contain a lower ratio of ceramic powder.
This arrangement has the disadvantage that insufficient joining strength results at the joining interface between the two compositions, especially in the center. One cause for this is now believed to be that relatively little slip occurs at the interface between the layers during forging. Also, since each layer is constructed from the same matrix material, it is not possible for a lower portion (forming the skirt) to constitute a material which is easily formed, and for a top portion (forming the head) to have high hardness, heat resistance and the like.
In a third known arrangement, a head portion of the piston is constructed of forged powdered metal or fiber reinforced metal, the skirt portion is made of an aluminum alloy casting, and the two portions are welded together. When the two parts are welded together, however, a brittle alloy layer is produced in the welded portion, contributing to low joining strength. Also, in the area of the weld, the basic characteristics of the powdered metal, that of high fatigue strength, proof strength and hardness, are lost. When the joining occurs by friction welding, burrs are produced in the welded portion. These burrs can cause stress concentrations and must be removed. However, the removal of the burrs is made difficult, at least on the inside of the piston, because the piston's irregular shape. Also, when the head portion is constructed of FRM, stress concentrations occur on the interface between the reinforcing materials, such as whiskers and short fibers, and the matrix. As a result, insufficient fatigue strength is provided at high temperatures.
An improved piston and method of constructing a piston are desired.